Synergistic chemomechanical dynamics of feedback-controlled microreactors

The experimental control of synergistic chemomechanical dynamics of catalytically active microgels (microreactors) is a key prerequisite for the design of adaptive and biomimetic materials. Here, we report a minimalistic model of feedback-controlled microreactors based on the coupling between the hysteretic polymer volume phase transition and a volume-controlled permeability for the internal chemical conversion. We categorize regimes of mono- and bistability, excitability, damped oscillations, as well as sustained oscillatory states with tunable amplitude, as indicated by experiments and representable by the FitzHugh-Nagumo dynamics for neurons. We summarize the features of such a colloidal neuron in bifurcation diagrams with respect to microgel design parameters, such as permeability and relaxation times, as a guide for experimental synthesis.Comment: 6 pages, 3 figure

Tracer dynamics in polymer networks: generalized Langevin description

Tracer diffusion in polymer networks and hydrogels is relevant in biology and technology, while it also constitutes an interesting model process for the dynamics of molecules in fluctuating, heterogeneous soft matter. Here, we study systematically the time-dependent dynamics and (non-Markovian) memory effects of tracers in polymer networks based on (Markovian) implicit-solvent Langevin simulations. In particular, we consider spherical tracer solutes at high dilution in regular, tetrafunctional bead-spring polymer networks, and control the tracer-network Lennard-Jones (LJ) interactions and the polymer density. Based on the analysis of the memory (friction) kernels, we recover the expected long-time transport coefficients, and demonstrate how the short-time tracer dynamics, polymer fluctuations, and the viscoelastic response are interlinked. Further, we fit the characteristic memory modes of the tracers with damped harmonic oscillations and identify LJ contributions, bond vibrations, and slow network relaxations, which enter the kernel with an almost linear scaling with the LJ attractions. This procedure proposes a reduced functional form for the tracer memory, allowing for a convenient inter- and extrapolation of the memory kernels. This leads eventually to highly efficient simulations utilizing the generalized Langevin equation (GLE), in which the polymer network acts as an additional thermal bath with tuneable intensity.Comment: 14 pages, 7 figures. Final version be published (open access) in The Journal of Chemical Physic

OLEDs as models for bird magnetoception: detecting electron spin resonance in geomagnetic fields

Certain species of living creatures are known to orientate themselves in the geomagnetic field. Given the small magnitude of approximately 48 mu T, the underlying quantum mechanical phenomena are expected to exhibit coherence times in the microsecond regime. In this contribution, we show the sensitivity of organic light-emitting diodes (OLEDs) to magnetic fields far below Earth's magnetic field, suggesting that coherence times of the spins of charge-carrier pairs in these devices can be similarly long. By electron paramagnetic resonance (EPR) experiments, a lower bound for the coherence time can be assessed directly. Moreover, this technique offers the possibility to determine the distribution of hyperfine fields within the organic semiconductor layer. We extend this technique to a material system exhibiting both fluorescence and phosphorescence, demonstrating stable anticorrelation between optically detected magnetic resonance (ODMR) spectra in the singlet (fluorescence) and triplet (phosphorescence) channels. The experiments demonstrate the extreme sensitivity of OLEDs to both static as well as dynamic magnetic fields and suggest that coherent spin precession processes of coulombically bound electron-spin pairs may play a crucial role in the magnetoreceptive ability of living creatures

Floquet spin states in OLEDs

Electron and hole spins in organic light-emitting diodes constitute prototypical two-level systems for the exploration of the ultrastrong-drive regime of light-matter interactions. Floquet solutions to the time-dependent Hamiltonian of pairs of electron and hole spins reveal that, under non-perturbative resonant drive, when spin-Rabi frequencies become comparable to the Larmor frequencies, hybrid light-matter states emerge that enable dipole-forbidden multi-quantum transitions at integer and fractional g-factors. To probe these phenomena experimentally, we develop an electrically detected magnetic-resonance experiment supporting oscillating driving fields comparable in amplitude to the static field defining the Zeeman splitting; and an organic semiconductor characterized by minimal local hyperfine fields allowing the non-perturbative light-matter interactions to be resolved. The experimental confirmation of the predicted Floquet states under strong-drive conditions demonstrates the presence of hybrid light-matter spin excitations at room temperature. These dressed states are insensitive to power broadening, display Bloch-Siegert-like shifts, and are suggestive of long spin coherence times, implying potential applicability for quantum sensing

Perdeuteration of poly[2-methoxy-5-(2'- ethylhexyloxy)-1,4-phenylenevinylene] (d-MEH-PPV): control of microscopic charge-carrier spin–spin coupling and of magnetic-field effects in optoelectronic devices

Control of the effective local hyperfine fields in a conjugated polymer, poly[2-methoxy-5-(2 '-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), by isotopic engineering is reported. These fields, evident as a frequency-independent line broadening mechanism in electrically detected magnetic resonance (EDMR) spectroscopy, originate from the unresolved hyperfine coupling between the electronic spin of charge carrier pairs and the nuclear spins of surrounding hydrogen isotopes. The room temperature study of effects caused by complete deuteration of this polymer through magnetoresistance, magnetoelectroluminescence, coherent pulsed and multi-frequency EDMR, as well as inverse spin-Hall effect measurements, confirm the weak hyperfine broadening of charge-carrier magnetic resonance lines. As a consequence, we can resolve coherent charge-carrier spin-beating, allowing for direct measurements of the magnitude of electronic spin-spin interactions. In addition, the weak hyperfine coupling allows us to resolve substantial spin-orbit coupling effects in the EDMR spectra, even at low magnetic field strengths. These results illustrate the dramatic influence of hyperfine fields on the spin physics of organic light-emitting diode (OLED) materials at room temperature, and point to routes to reaching exotic ultra-strong resonant-drive regimes in the study of light-matter interactions

Perdeuterated Conjugated Polymers for Ultralow‐Frequency Magnetic Resonance of OLEDs

The formation of excitons in OLEDs is spin dependent and can be controlled by electron-paramagnetic resonance, affecting device resistance and electroluminescence yield. We explore electrically detected magnetic resonance in the regime of very low magnetic fields (<1 mT). A pronounced feature emerges at zero field in addition to the conventional spin-1/2Zeeman resonance for which the Larmor frequency matches that of the incident radiation. By comparing a conventional pi-conjugated polymer as the active material to a perdeuterated analogue, we demonstrate the interplay between the zero-field feature and local hyperfine fields. The zero-field peak results from a quasistatic magnetic-field effect of the RF radiation for periods comparable to the carrier-pair lifetime. Zeeman resonances are resolved down to 3.2 MHz, approximately twice the Larmor frequency of an electron in Earth's field. However, since reducing hyperfine fields sharpens the Zeeman peak at the cost of an increased zero-field peak, we suggest that this result may constitute a fundamental low-field limit of magnetic resonance in carrier-pair-based systems. OLEDs offer an alternative solid-state platform to investigate the radical-pair mechanism of magnetic-field effects in photochemical reactions, allowing models of biological magnetoreception to be tested by measuring spin decoherence directly in the time domain by pulsed experiments